1. Field of the Invention
The present invention relates to a photosensitive member for electrophotography composed of a thick photosensitive layer, which is excellent in wear resistance and electrical characteristics, and can form good copy images without image-defects and image-roughness.
2. Description of the Prior Art
Known photosensitive materials for forming a photosensitive layer include inorganic photoconductive materials such as selenium, cadmium sulfide or zinc oxide. These photosensitive materials have many advantages such as low loss of charges in the dark, an electrical charge which can be rapidly dissipated with irradiation of light and the like. However, they have disadvantages. For example, a photosensitive member based on selenium is difficult to produce, has high production costs and is difficult to handle due to inadequate resistivity to heat or mechanical impact. A photosensitive member based on cadmium sulfide has defects such as its unstable sensitivity in a highly humid environment and loss of stability with time because of the deterioration of dyestuffs, added as sensitizer, by corona charge and fading with exposure. These photosensitive members have also a problem from the viewpoint of safety.
Many kinds of organic photoconductive materials such as polyvinylcarbazole and the similar compounds have been proposed for forming an organic photosensitive layer. These organic photoconductive materials have superior film forming properties, are light in weight, etc., but inferior in sensitivity, durability and environmental stability compared to the aforementioned inorganic photoconductive materials.
Various studies and developments have been in progress to overcome the above noted defects and problems. A function-divided organic photosensitive member of a laminated or a dispersed type has been proposed, in which a charge generating function and a charge transporting function are shared by different compounds. In usual, a photosensitive layer in the function-divided photosensitive member of the laminated type is composed of a charge generating layer containing an organic charge-generating material, a charge transporting layer containing an organic charge-transporting material and a binder resin. A photosensitive layer in the function-divided photosensitive member of the dispersion-type is composed of an organic charge-generating material and an organic charge-transporting material which are dispersed in a binder resin.
Such a function-divided organic photosensitive member can display performances excellent in electrophotographic properties such as chargeability, sensitivity, residual potential, durability with respect to copy and repetition, because most adequate materials can be selected from various materials. Moreover, function-divided photosensitive members have high productivity and low costs, since they can be prepared by coating, and suitably selected charge generating materials can freely control a region of photosensitive wavelength.
In particular, the function-divided photosensitive member of the dispersion-type can be used as a positively chargeable photosensitive member. The positively chargeable photosensitive member generates a little ozone and has an environmental resistance compared to a negatively chargeable photosensitive member. Therefore the function-divided photosensitive member of the dispersion-type has been paid to attention.
However, the organic photosensitive member is generally poor in mechanical strength and durability compared to the inorganic photosensitive member. A thickness of the organic photosensitive member decreases with its friction against toner, paper, a cleaning member and other similar loads in the copying machine. A layer-decreasing degree caused by wear depends on materials and mechanical systems, but generally it is 0.2-1 .mu.m after 10,000 times of copy. The decrease of the layer-thickness causes the deterioration of chargeability. When the deterioration is beyond tolerance limits, the lifetime of the photosensitive member is over. As a result, the organic photosensitive member is poor in resistance to copy.
Therefore, it has been developed and researched that a photosensitive layer is made thick in order to improve durability and to make the lifetime of the photosensitive member long.
However when the thickness of a photosensitive layer is merely made thick, electrical charges accumulate in the photosensitive layer after the repetition use, resulting in remarkable increase of residual potential, deterioration of chargeability and image-disorders.